Batteries and more batteries

Manuals

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The main battery bank, 4 x 120Ah Gel cells (2010-09-12 11:46:39 PENTAX Optio WS80 [f/3.8, 1/60s] ISO 400) Jeanneau 57 Bank 1

The additional bank of 4 x 120Ah batteries (2010-09-12 12:19:10 PENTAX Optio WS80 [f/3.8, 1/60s] ISO 400) Jeanneau 57 Bank 1

Zanshin has lots of batteries on board, see the top right chart on this page for a list. Don't let the different voltages confuse, it is a simple conversion from 120Ah capacity at 12V to 60Ah at 24V. In actuality the electrical chemical cell that produces power only generates around 2V and 6 of these are always combined to make a 12V battery. So I've got a total of 1320Ah @ 12V aboard, that is the equivalent of about 20 car batteries! The capacity numbers are how many amps are stored in the battery when discharging over a period of 20 hours, if the discharge rate is higher then the total amps available goes down very quickly. The house bank will last 20 hours when 30amps are removed constantly, this is 720Watts of energy (a hair drier or espresso machine uses about 1800W). If I were to leave that hair drier running the batteries would be flat in about 3 hours.

The rate at which electricity can be put back into the batteries depends upon a number of factors, the major one being the battery type. Normal car batteries are so-called “Flooded” lead-acid and they usually are limited to charging at 20% of the 20-hour capacity. The batteries aboard Zanshin are gel-cells, meaning that there is no liquid sloshing around inside the battery anymore (which has the advantage that there is no danger of spilling acid) but that the electrolyte is fixed in place. The advantage to using this technology is that internal resistance is lowered and putting power into the battery doesn't generate much heat and therefore the charging amperage can be increased. The Optima batteries have (in theory) no upper limit, but the manuals suggest limiting amperage to 4 times the C/20 rating. The Dryfit batteries can be charged at 50% of their C/20 rating - more than double as fast as normal lead-acid batteries; halving charging time.

In order to keep the batteries working over a long period of time it is recommended that they never get discharged below 50% of their capacity. In addition, charging over 90% takes disproportionately long, so in reality my huge 480Ah/24V battery bank only contains 190Ah/24V of usable power in normal conditions when underway. Looking at the consumption table to the right, this is pretty much what I am using daily while sailing. Due to charging losses we need about 25% more power to charge the bank, so the energy deficit is about 300Ah. Luckily I have several means of charging the bank, either through the engine alternator or wind generator - or by using the generator to power the Victron inverter/charger which will put up to 120Amp into the battery bank. At the same time, the boat's 2 chargers are also loading at 60Amps each - a grand total of 240Amps are now being put back into the battery bank and using about 6Kw of power. Since the generator is rated at 9.5Kw this leaves ample reserves while putting the generator under a good load (diesels don't like to run without a load). This formula means that I could run the generator for about 1:15 per day in order to keep up with energy demands.